The present invention relates to the Highway Addressable Remote Transducer (“HART”) system, and more specifically to equalizing cable losses in a C8PSK HART system.
The HART protocol is a global standard for sending and receiving digital information across analog wires between intelligent field devices and a control system, and enhancing traditional 4-20 mA signaling by simultaneously allowing two way digital communications. FIG. 1 illustrates a schematic diagram of a HART network. As shown, a HART network 100 includes a control room 101 and a number of field devices 110.1-110.n coupled to the control room 101. The field devices typically are instruments controlled by the control room 101, e.g., pressure sensors, control valves, etc. They collect information in response to a command from the control room 101 and communicate the information back to the control room 101 via the HART protocol. The control room 101 then controls the field device, e.g., 110.1, according to the information received from that field device. With the HART protocol, the control room 101 communicates with field devices to carry out device configuration or reconfiguration, device diagnostics and device troubleshooting without sending an engineer to the field. In FIG. 1, R1-Rn and C1-Cn represent resistance and capacitance of communication lines interconnecting the field devices 110.1-110.n to the control room 101 respectively, and Rsense represents the load of the control room 101 to the network.
Coherent 8-way Phase Shift Keying (C8PSK) is widely used in the telecommunication industry and is known for providing the best balance between faster communications and lower power consumption required for 2-wire field devices.
FIG. 2 illustrates a schematic architecture of a transmitter in a prior art C8PSK HART field device. As shown, the input of a symbol mapping device 201 is coupled to the input of a transmitter 200. The symbol mapping device 201 receives incoming data stream at 9.6 Kbits per second, and divides the data stream into symbols, each of which is a group of three consecutive bits (tribit) and is mapped into a phase angle. The frequency of the output signal of the symbol mapping device 201 is 3.2 KHz.
The input of a look up table (LUT) 202 is coupled to the output of the symbol mapping device 201 to receive the symbols. A symbol is used as a pointer of the LUT 202 to find out the symbol's real and imaginary parts.
A root raised cosine (RRC) filter 203 is coupled to the output of the LUT 202 to receive the imaginary part (Imag) of a symbol, and an RRC filter 204 is coupled to the output of the LUT 202 to receive the real part (Real) of the symbol. The RRC filters 203 and 204 filter the real and imaginary parts of the symbol and output a quadrature phase signal Q and an inphase signal I of the symbol respectively.
One input of a mixer 205 is coupled to the output of the RRC 203 to receive the quadrature phase signal Q, and another input of the mixer 205 receives a carrier cos(ωt). The mixer 205 multiplies Q and cos(ωt).
One input of a mixer 206 is coupled to the output of the RRC 204 to receive the inphase signal I, and another input of the mixer 206 receives a carrier sin(ωt). The mixer 206 multiplies I and sin(ωt).
One input of an adder 207 is coupled to the output of the mixer 205 to receive the signal Q×cos(ωt), and another input of the adder 207 is coupled to the output of the mixer 206 to receive the signal I×sin(ωt). The adder 207 combines these two signals together and outputs Q×cos(ωt)+I×sin(ωt), the signal to be transmitted.
A digital to analog converter (DAC) 208 has an input coupled to the output of the adder 207, and converts the signal to be transmitted, Q×cos(ωt)+I×sin(ωt), to an analog signal.
An input of a buffer 209 is coupled to the output of the DAC 208. The analog signal is buffered at the buffer 209 before entering the transmit channel.
The channel characteristics are different for each field device in a HART network, dependent on their distances from the control room. In prior art C8PSK HART systems, the control room does not have data representing the field devices' location and hence does not know the bandwidth of the cable between it and a field device. The cable bandwidth could cause large bit error rates at a control room receiver due to the no-linearity of the cable system.
According to the C8PSK HART protocol, bandwidth limitations of a channel from a control room to a field device can be estimated and equalized by a channel estimation device in a receiver in the field device. However, neither the C8PSK HART standard nor prior art C8PSK HART field devices have transmitter side equalization.
Therefore, it would be desirable to provide a method to adjust the signal from a field device to overcome cable bandwidth limitations and a field device which can compensate for the bandwidth limitations in its transmit channel, and to allow good reception in the control room irrespective of where the field device is in the network.